Off heat, coat grill with cooking spray. Prepare gas grill for direct, high-heat grilling or build a high-heat charcoal bed in the center of the coal grate on a charcoal grill.

Divide the chicken, zucchini and onion among four large skewers, threading the meat and vegetables over the pointy ends and down the skewers.

Set the skewers directly over the heat and grill for 16 minutes, turning about every 4 minutes onto each of the other three sides, so that all four sides are well browned. When turning the skewers, use tongs to gently grasp some of the meat at the center. Do not turn the skewers by their ends — or put your hands down near the heat source. Yields 1 skewer per serving.